The present invention is directed to pressure sensitive electrical switches and, more particularly, to switches of the type having a snap element which changes shape rapidly at a predetermined pressure and causes the switching state to be altered.
Snap action elements are particularly suited for use with pressure sensitive switches since a force proportional to the pressure of fluid may be conveyed to a snap element by a simple linkage mechanism. The snap element will change shape rapidly after a desired threshold force is applied but will distort only minimally until the threshold level is reached. The snap element may be coupled to switch contacts in a variety of ways such that sharp make or break actuation will result. Generally, once the threshold force level is reached and the snap element "snapped", the force applied to the element will have to be reduced substantially below the threshold level before the element will snap back to its original shape.
A numbered of pressure sensitive electrical switches have utilized snap elements and provided for adjustment of precise actuation pressures by means of auxiliary spring elements which act on the snap elements in opposition to the force applied from the pressurized fluid. U.S. Pat. No. 3,535,480, issued Oct. 20, 1970, to Bahniuk; U.S. Pat. No. 3,335,242, issued Aug. 8, 1967, to Johnson; and, U.S. Pat. No. 3,773,991, issued Nov. 20, 1973, to Krieger et al, are all of this type. The pressure sensitive switch shown in U.S. Pat. No. 3,573,410 issued Apr. 6, 1971, to Budzich et al, is somewhat similar in design except that the snap element is bistable and the spring member is not adjustable. In U.S. Pat. No. 3,553,402, issued Jan. 5, 1971, to Hire, a similar switch is shown in which the adjustable spring affects the snap action in only one direction.
In U.S. Pat. No. 3,890,477, issued June 17, 1975, to Saunders, means are provided to alter the shape of the snap element slightly in a pressure sensitive switch and thus change the force level required to cause actuation.
U.S. Pat. No. 2,704,551 issued Mar. 22, 1955, to Ralston, shows a pressure sensitive valve mechanism with a snap element controlling opening and closing of the valve. The snap element shown in this device is generally in the shape of a truncated cone with the distorting force applied along the edge of the narrowed truncated portion.
As shown in U.S. Pat. No. 3,676,817, issued July 11, 1972, to Bletz, and U.S. Pat. No. 3,602,863, issued Aug. 31, 1971, to Place, snap elements have been used with temperature sensitive electrical switches as well. Additionally springs are typically used in such devices to adjust the temperature at which the bi-metal snap mechanisms distort.
Heretofore, to alter the threshold pressure for actuation of a snap element type of pressure sensitive electrical switch, a different snap element having a different force level requirement could be inserted in the switch or, alternatively, an adjustable, auxiliary spring mechanism could be provided. Neither alternative has proven to be completely acceptable. Snap elements which are dimensionally interchangeable but which differ in operating characteristics are not readily available for a large range of operating forces. The use of an auxiliary spring mechanism may adversely affect the snapping action of the snap element.